Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines a measurement wavefront reflected from the surface of interest with a reference wavefront reflected from a reference surface to produce an interferogram. Fringes in the interferogram are indicative of spatial variations between the surface of interest and the reference surface.
A scanning interferometer scans the optical path length difference (OPD) between the reference and measurement legs of the interferometer over a range comparable to, or larger than, the coherence length of the interfering wavefronts, to produce a scanning interferometry signal for each camera pixel used to measure the interferogram. A limited (or “low”) coherence length can be produced, for example, by using a broadband light source (e.g., a white light source), which is referred to as scanning white light interferometry (SWLI). A typical SWLI signal is a few fringes localized near the zero optical path length difference (OPD) position. The signal is typically characterized by a sinusoidal carrier modulation (the “fringes”) with bell-shaped fringe-contrast envelope. The conventional idea underlying SWLI metrology is to make use of the localization of the fringes to measure surface profiles. Low-coherence interferometry signals can also be produced with narrow band light that illuminates an object over a wide range of angles, such as, for example, in imaging interferometers that have a high numerical aperture.
Techniques for processing low-coherence interferometry signals include two principle trends. The first approach is to locate the peak or center of the envelope, assuming that this position corresponds to the zero optical path length difference (OPD) of a two-beam interferometer for which one beam reflects from the measurement surface. The second approach is to transform the signal into the frequency domain and calculate the rate of change of phase with wavelength, assuming that an essentially linear slope is directly proportional to object position. See, for example, U.S. Pat. No. 5,398,113 to Peter de Groot, the entire contents of which are incorporated herein by reference. This latter approach is referred to as Frequency Domain Analysis (FDA).
Unfortunately such assumptions may break down when applied to a measurement object having a thin film because of reflections by the top surface and the underlying film/substrate interface. Recently a method was disclosed in U.S. Pat. No. 6,545,763 to S. W. Kim and G. H. Kim to address such structures. The method fit the frequency domain phase profile of a SWLI signal for the thin film structure to an estimated frequency domain phase profile for various film thicknesses and surface heights. A simultaneous optimization determined the correct film thickness and surface height.
One useful application of such surface profiling techniques is the profiling of lithography wafers covered with photoresist. The goal in this case is to determine the topography of the top surface of the photoresist over a patterned substrate and to provide information about the location of this surface with respect to some system datum, one application of which is to establish the position of best focus during the photolithographic process. See, for example, commonly owned U.S. Patent Application Publication No. 2005/0057757 A1 entitled “LOW COHERENCE GRAZING INCIDENCE INTERFEROMETRY SYSTEMS AND METHODS” by Xavier Colonna de Lega et al., the contents of which are incorporated herein by reference.